1. Field of the Invention
This invention relates to a process for producing a hydrated and cured product of a lime-gypsum-coal ash mixture as a gas-purifying agent. More particularly is related to a process for producing a highly effective desulfurizing agent.
2. Description of the Related Art
Removal of sulfur oxides discharged from heavy oil combustion or coal combustion boilers provided in thermal power stations has been carried out according to a wet process (e.g. limestone-gypsum process) or a dry process. However, development of a simplified and economical desulfurization process in place of the above-mentioned processes has been desired.
On the other hand, in the case of coal combustion boilers, an enormous quantity of coal ash has been discharged and a part thereof has been utilized as a material to be incorporated into cement or for land reclamation, but in order to achieve a higher level utilization of the coal ash, its utilization for a dry desulfurizing agent has been developed by the present inventors (Japanese patent application laid-open No. Sho 61-209038/1986).
The desulfurizing agent making use of coal ash is produced basically by adding water to a raw material mixture consisting of slaked lime, gypsum and coal ash, followed by heating the resulting slurry in steam atmosphere to hydrate and cure it, and subjecting the resulting material to grinding treatments, classifying and drying (one step cure process, see FIG. 2). Thus obtained desulfurizing agent is of a porous and hardened material and has a specific feature of fixing SO.sub.2 contained in exhaust gases into a chemically stable CaSO.sub.4 to remove it, as shown in the following formula (I): EQU X.multidot.Ca(OH).sub.2 +SO.sub.2 =1/2O.sub.2 =CaSO.sub.4 +X+H.sub.2 O.uparw. (1)
wherein X refers to constituents of the agent other than Ca(OH).sub.2.
However, in order to bring a dry desulfurization process into practical use, it is very important to establish a technique of producing a highly active desulfurizing agent in a large scale and with a high yield.
In reference to the above process, there has been developed a process of once heating the raw material mixture to form a hydrated and cured one, followed by roughly grinding it, granulating the resulting ground material and subjecting this material to a secondary cure, thereby improving the yield of product and also shorten the period of time required for production (Japanese patent application laid-open No. Sho 62-254824/1987, two step cure process, see FIG. 3). However, the desulfurization performance of the resulting agent has been somewhat inferior to that in the case of the above one step cure process as a basic production process. Further, in the aspect of a production process, a more simplified process is desired.